Switched-boost Action: A phenomenon forachieving Time-division-multiplexed Multi-port

Power Transfer for nano grid applicationsOlive Ray and Santanu Mishra

Department of Electrical EngineeringIndian Institute of Technology Kanpur, India

olive@iitk.ac.in, santanum@iitk.ac.in

Abstract—Multi-port power converter topologies are used tointerface multiple terminals (source or load) using a singleconverter topology. In this paper, the possibility of using switchnode of a power converter stage as interface to multiple outputstages has been investigated. This interfacing of multiple outputsat the same switch node occurs using time-division multiplexingof the available input power. This phenomenon is denoted as‘switched-boost action’ and the concept has been illustrated usingthe boost converter topology. The proposed approach has beenused to interface the boost-derived architectures to different typesof outputs- ac, dc and isolated dc. The circuit operation has beenvalidated using experimental results. These circuits have beenproposed as possible candidates for nano grid applications.

Index Terms—Switched-boost action, Time-division multiplex-ing, Boost converter, Multi-port converters.

I. INTRODUCTION

The widespread proliferation of low power rated powerelectronic loads in residential applications has resulted inwidespread research in the area of multi-port power convertertopologies. The major motivation behind research in this di-rection lies in the fact that reducing the number of stages mayresult in lower number of components, higher compactness andbetter coordination control of the overall system. A typicalcharacteristic behavior of the loads in residential system isthat they can be either ac- or dc- based. For e.g. the lightingloads such as CFL, LED lights require dc input supply, whilemotor loads require ac voltages. A cluster of such loadsin a residential system may comprise of a nano grid. Thepower rating for the overall system lies typically within 1 kW.The scope of the work presented here would describe powerconverters which can be used in nano grid applications [1]-[4].

Multi-port power converter topologies reported in literaturecan be classified into two broad categories: isolated and non-isolated. In both of the two cases, the nature of the outputswhich can be obtained from the topologies are same in nature-either all are dc or ac. For example, the most common meansfor achieving multiple ac outputs is by using a multi-windingarrangement. For achieving multiple dc outputs, the multi-winding arrangement is followed by a conversion stage togenerate dc outputs. In this context, in order to obtain dcand ac outputs simultaneously, the most common approachis to use additional cascaded dc-ac stages to multi-port dc-dcconverters. The converter topologies described in this paper

can be used to achieve simultaneous dc and ac outputs usinga single architecture.

Within the scope of the work, multi-port power convertertopologies have been studied which use the switch nodevoltage to interface different converter topologies. This methodof interfacing has been used to generate dual dc or dc-ac (hybrid) outputs from a single system using a singlearchitecture. The paper is organized as follows: Section IIdescribes the concept of time division multiplexing and itspresence within power transfer in power electronic converters.The ‘switched-boost action’ has been introduced in SectionIII. The phenomenon has been utilized to realize multi-portpower conversion stages. Section IV describes case studiesconcerning the proposed circuit modification. This is followedby experimental validation of the concept. Section V concludesthe paper.

II. TIME-DIVISION MULTIPLEXING IN POWERCONVERSION

The control objectives of any power electronic convertertopology are achieved by exerting suitable control over a con-trol variable [5]. This control variable may be duty ratio in thecase of dc-dc converter or modulation index in the case of aninverter. Since the power electronic converter operates throughdifferent switching modes within each switching interval, thereexists a switching state which corresponds to the operation ofcontrol variable and which corresponds to the energy transferfrom the input side. Fig. 1 illustrates the above fact usingbuck converter (Fig. 1 (a)), boost converter (Fig. 1 (b)) andthe voltage source inverter topology (Fig. 1 (c)).

The buck converter [Fig. 1 (a)] operates through two switch-ing states- S1,buck being either ‘on’ or ‘off’. The time durationwhen this switch is ‘on’ is decided by the duty ratio for theswitch and this controls the output voltage for the buck output.As shown in the operating waveforms for the circuit in Fig.1 (a), the input current for the buck converter topology isdiscontinuous, and current is drawn from the source wheneverthe top-switch is ‘on’. The switch node voltage vsn,buck isequal to the input voltage during this interval. It has to benoted here during the rest of the time period when the topswitch is ‘off’, the voltage at the input is not required forconverter operation as power is not drawn from the input side.

Fig. 1. Power converter topologies and their relation with the control variables. (a) buck, (b) boost, and (c) voltage source inverter.

Fig. 1 (b) shows the boost converter topology, where thecontrol switch Sboost regulates the state variables of the con-verter. The gate signal (GSboost) and the corresponding switchnode voltage waveform (vsn,boost) for the boost converter havebeen shown in Fig. 1 (b). For the boost converter the inputcurrent is continuous. However, the current builds up duringthe switch state when the control switch is ‘on’. This is theinterval when energy is drawn from the source. During theremaining portion of the switching period, the stored inductorenergy is discharged into the dc output.

Fig. 1 (c) shows the voltage source inverter topology.In this case, the control variable is the modulation index.For the inverter, the input current is discontinuous and theduration for which the inverter draws current from the source isproportional to its modulation index. Based upon the analysisof the above three cases, the following observations can bemade:

A. Voltage-fed converters

1) Voltage-fed converters such as buck converter, voltagesource inverter operate by drawing a discontinuouscurrent from the source. The input voltage source isrequired by the converter topology only during thisinterval. The time duration for this interval is either dutyratio or the modulation index. This interval is denotedas the ‘power interval’ for the voltage-fed topologies.

2) The value of the input voltage source does not affect theoperation of the voltage-fed topologies during the non-

power interval. This interval is denoted as ‘zero interval’.During this interval, since the source remains isolated bythe open-switches from the output, its magnitude doesnot affect the behavior of the circuit, and thus, it can bezero.

3) From the source end, the operating states of the voltage-fed topologies can thus be thought of as if the inputpower is multiplexed into the converter during the powerinterval only.

B. Current-fed topologies

1) For a current-fed topology, e.g. boost converter, thepower transfer operation occurs during the free-wheelperiod when the input power is discharged into theoutput end. The switch node voltage remains equal tothe output voltage during this interval.

Based upon the above observations, the concept of switchedboost action will be presented in the next section.

III. SWITCHED-BOOST ACTION

A. Scope for pulse-type dc input for voltage-fed converters

The analysis of voltage-fed topologies indicates that theinput voltage is required for converter operation during thepower intervals. Since the value of the input voltage is notimportant for converter operation during the zero interval,a pulsed voltage waveform (vpulse) switching between thevoltage values {0, Vdcin} can also be used as the input to

Fig. 2. Pulse-voltage waveform instead of voltage-stiff dc input used for thebuck topology.

Fig. 3. Switch-node voltage of boost converter used as the source for voltage-fed converter topologies.

the converter instead of having a stiff voltage source having adc value (Vdcin). It has to be mentioned here that the durationof the pulse-voltage waveform in high state should be greateror equal to the power interval of the voltage-fed topologies.This has been illustrated using Fig. 2 for the buck convertertopology.

B. Boost converter switch-node as the pulsed-dc input source

The pulse-voltage waveform can be generated by a wide va-riety of methods. In this work, the boost converter topology hasbeen used to derive the pulse-voltage input source. As shownin Fig. 1 (b), depending upon the state of the controllableswitch, the switch node voltage assumes the value of ‘zero’or ‘vdcout’. This switch node voltage thus has a pulsed-natureand can be used as a source to the voltage-fed topologies. Theconcept of using the switch node of the boost converter as theinput to voltage-fed topologies has been illustrated using Fig.3. In this instance, the switch node voltage vsn,boost is usedas the interface to the voltage-fed topologies. Two possibleconfigurations which have been studied in this work has beenillustrated using Fig. 4.

C. Characteristics of Switched-boost action

The above mentioned phenomenon in which the switch nodeof a dc-dc conversion stage is used to interface voltage-fedconverters is denoted as ‘switched-boost action’ in this work.This switched-boost action in any topology is characterized bythe following characteristics:

1) The input inductor current of the boost stage gets sharedbetween the freewheeling diode current and the powerinterval current drawn by the interfaced circuit during

Fig. 4. Voltage-fed topologies e.g. buck converter and the voltage sourceinverter interfaced at the switch node of the boost converter topology.

Fig. 5. Integration of boost and buck converter at the switch node.

the freewheeling interval. Thus the switched node is thesource of the input power for the interfaced converter.The diode current appears as a notched waveform com-pared to that in a corresponding dc-dc stage.

2) Although the input current is supplied from the inputboost inductor, the voltage across the interfaced con-verter is equal to the boost output voltage to which theswitch node gets clamped. It has to be mentioned herethat the switch node voltage should remain clamped tothe output dc voltage so long as the power interval existsfor the interfaced voltage-fed converter.

The following are the advantages of using switch-boostaction in order to generate multi-output converters:

1) Converters based upon this principle operate using lowernumber of active switches compared to the separateconverters. When operating in CCM the characteristicsof the integrated topology is same as that of individualconstituent converters. This has been illustrated usingFig. 5 and Fig. 6. In this instance, the buck converteris interfaced at the switch node vsn,boost [see Fig. 5].It can be seen that this interfacing results in the switchpair of S1,buck and S2,buck being in parallel to the boostswitch S1,boost. The corresponding operating regionsof each of the switches have been shown in Fig. 6.Since the operating regions of these three switches donot contradict each other, one of the switches can bereduced. The resultant topology is the integrated dual-output converter which will be described in the nextsection.

Fig. 6. Switch operating regions of the integrated converter. (a) Boost, (b) and (c) buck.

2) A characteristic feature of voltage-fed converters withsynchronous switching is that both the switches musthave a dead-time in order to avoid shoot-through. Themajor reason for this is that these topologies are in-terfaced to a voltage-stiff source and hence protectionfeatures must be in place to prevent shorting of thesource. In the present approach, the switch networkis interfaced at the switch node and is separated bythe voltage-stiff dc source by the inductor. Hence theresultant topology is inherently shoot-through protected.

3) Since both topologies are controlled using the same setof switches, the control becomes easier. The overallsystem has higher power processing density.

The major operational constraints and design challenges forsuch converters are as follows:

1) The switched-boost action requires the switch node tobe clamped to the output dc voltage during the power in-tervals. Hence this is a major constraint in the operationof the converter and it is reflected by having a minimumdc load at the boost output.

2) Since the total time period is shared by both the boostduty cycle and the power interval, their sum should beless than the total time period.

3) Pulse-width modulation used for conventional converterscannot be used for operation of these converters. Hence,newer PWM control strategies need to be devised forsuch converters.

D. Higher order converter applications of Switched-boostaction

The boost converter is limited in the maximum achievablegain due to the presence of non-idealities in the circuit ele-ments. The switch nodes of higher order converter topologiescan also be used for interfacing converter topologies. Theadvantage for this is higher order converters have high gains.Thus freewheel period corresponding to the step-up stage has ahigher window in such cases. Fig. 7 shows the switched-boostinverter [6] topology which uses the proposed switched-boostaction with the Inverse-Watkins Johnson converter topology.The shaded portion in the figure denotes the switched-boostaction elements.

The next section will highlight case studies of the convertertopologies derived from the boost converter and utilizing

Fig. 7. Switched-boost inverter topology.

Fig. 8. Boost-derived hybrid converter.

the switched-boost action. These converters are denoted asintegrated power converter topologies in the work.

IV. CASE STUDIES

A. Boost-derived hybrid converter (BDHC): Interfacing avoltage source inverter at the switch node of the boost topology

1) Overview: The boost-derived hybrid converter is ob-tained by interfacing the full-bridge H-bridge inverter topologyat the switch node of the boost converter topology. Theconverter topology, shown in Fig. 8, has been studied in [7].The operation of the converter topology requires the use of twocontrol variables: the duty cycle DST and the modulation in-dex ma. The inverter output is regulated using the conventionalsine-PWM control strategy. However, this switching schemeneeds to be incorporated within the switching strategy for theboost converter. Fig. 9 shows the modification required in thePWM switching scheme for the BDHC topology. The majorconstituents of the PWM switching block, shown in Fig. 9, arethe conventional unipolar sine PWM stage, boost PWM stage,

Fig. 9. PWM control scheme for boost-derived hybrid converter.

(a) (b)

Fig. 10. The BDHC produces a dc output (vdcout) as well as an ac output(vacout) from an input voltage (vdcin) of 48 V dc (Ch. 1). (a) dc outputof 75.4 V (Ch 4) and ac output of 30 V (rms) (Ch. 2) for DST = 0.4 andMa = 0.6 (b) dc output of 108 V dc (Ch 4) and ac output of 30 V (rms)(Ch. 2) for DST = 0.6 and Ma = 0.4.

(a) (b)

Fig. 11. Step-load change behavior of the BDHC under closed loop operation.Step change in (a)ac load, (b) dc load.

and the interfacing stage. The PWM scheme can provide aconstant duty cycle across all the switching intervals for boostdc output control. The PWM scheme has been studied in [8].

2) Operational behavior: The BDHC is capable of pro-viding dc and ac outputs simultaneously from a dc input. Amajor characteristic feature is that the gains obtained are sameas separate boost converter and an inverter. The operatingbehavior of the BDHC has been shown in Fig. 10. Fig. 10

Fig. 12. BDHC in a nano grid application.

(a) shows the behavior of the converter when the duty cycleof operation is 0.4 and the modulation index is 0.6. Note thatthe sum of duty ratio and modulation index is 1, which isthe limiting condition of the operating constraint describedin the previous section. Fig. 10 (b) shows the behavior ofthe converter when the duty cycle is raised to 0.6 while themodulation index is 0.4. The dc gain of the converter increaseswith the increase in the duty cycle similar to a conventionalboost converter. For an input dc voltage of 48 V dc, the dcoutput voltages of 75.4 V and 108 V dc are obtained. Sincethe sum of duty ratio and modulation index is the same in boththe cases the ac output voltage has the same value (30V acr.m.s.). The value is similar to what can be obtained using asingle phase inverter. Fig. 11 shows the closed loop behaviorof the BDHC to load changes in either output.

3) Application in nano grid scenario: The focus areabehind development of integrated converter topologies in thiswork is nano grid applications. Fig. 12 shows a typicalapplication area of the BDHC in a nano grid scenario [9].The representative schematic assumes a dc-based distributionpowered by both solar panel as well as utility grid. In such ascenario, the power converter topology must be bidirectionalin nature to interface with the grid. The interface betweenthe solar resource and the load requires a unidirectional dc-dcstage. The BDHC with its bidirectional ac port and dual-dcports can be used for this application. The integrated natureof the power converter makes the control for power flowmanagement easier.

B. Integrated dual-output converter (IDOC): Interfacing a dc-dc stage at the switch node of the boost converter

The integrated dual-output converter topology is achievedby interfacing the buck converter at the switch node of theboost converter topology. The schematic of the IDOC hasbeen shown in Fig. 13. The operational characteristic of theconverter topology has been studied in [10]. The converter canprovide a step-up and a step-down dc output from a single dcinput. The ranges of output voltages that can be achieved aresame as that of a conventional buck and a boost converter. Fig.14 shows the steady state operating behavior of the IDOC.

Fig. 13. Integrated dual-output converter

Fig. 14. Verification of steady state behavior of IDOC. Input voltage vin(Ch. 1), inductor current iL1 (Ch. 4), step-up vo1 (Ch. 3) and step-down vo2(Ch. 2) voltages for D1 = D2 = 0.33.

Fig. 15. Integrated dual-output converter in a nano grid scenario.

From a 12 V dc input, the dc outputs of 18 V and 6 V dc areobtained for duty cycles of 0.33 and 0.33 respectively. It has tobe mentioned here that there are two control variables denotedby the duty cycles for control of each of the outputs. Fig. 15shows the application of the IDOC in a nano grid scenario. Inthis case the nano grid operates through a dc bus and there isa battery storage unit for back up. The input to the nano gridis through a dc source. For this multi-port scenario the IDOCcan be used to interface the outputs.

C. Other topology extensions derived from the boost converterutilizing the switched-boost action

Isolated multi-port dc-dc converter topologies can be syn-thesized by interfacing the isolated voltage-fed convertertopologies at the switch node of the boost converter topology.The full-bridge isolated buck converter topology has beeninterfaced at the switch node of the boost converter to obtainthe integrated isolated dual-output converter topology (see Fig.

Fig. 16. Integrated isolated dual-output converter.

16) [11]. Analysis of the converter shows that the behavioris similar to boost and full bridge isolated buck convertertopology.

V. CONCLUSION

In this paper a review of dual-output converter topologieshave been made. These topologies have been synthesized byinterfacing a voltage-fed converter topology at the switch nodeof the boost converter. The fundamental principle behind theoperation of the converters is the switched-boost action. Thisphenomenon has been illustrated in this paper. Three casestudies have been presented in order to illustrate the proposedcircuit behavior.

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